1. Field of the Invention
The present invention relates to a wire electric discharge machine and more particularly to a wire electric discharge machine having a function of moving from an arbitrary retreat position in an electric discharge state and returning to a halt position.
2. Description of the Related Art
In prior art, a wire electric discharge machine has the following function: if a wire electrode and a workpiece are short-circuited during machining and electric discharge machining does not proceed, relative movement of the wire electrode and the workpiece is performed to make the wire electrode trace back a machining path and remove the short-circuit, and then the wire electrode is returned to an original machining position and the machining is restarted.
Japanese Patent Application Laid-Open No. 56-27741 discloses a technique in which, if electric discharge machining is halted due to a break in a wire electrode during program operation, the wire electrode is automatically returned to a prestored machining start point, wire connection operation is performed in a machining start hole, and returned from the machining start point to a machining halt point along a machined path by dry-run control, and machining is performed again at the machining halt point in an electric discharge state.
Japanese Patent Application Laid-Open No. 6-277949 discloses a technique in which, if a wire electrode and a workpiece are short-circuited during machining by a wire electric discharge machining device and electric discharge machining does not proceed, relative movement of the wire electrode and a table is performed to make the wire electrode trace back a machining path and remove the short-circuit (retraction control), and then electric discharging is started, the wire electrode is returned to an original machining position, and further electric discharge machining is continued from that area.
Japanese Patent Application Laid-Open No. 1-210221 discloses a technique in which, if a short-circuit occurs because of deviation of a machining start hole with respect to a machining start point, a wire electrode is manually retreated to a position in which the short-circuit can be removed and machining can be started, and the wire electrode is automatically returned to a true machining start point position from that area while electric discharge machining is performed.
FIG. 10 is a flowchart illustrating a flow of processing by wire electric discharge machining in prior art.
Movement is made by a G00 positioning instruction (step sd01). Execution of a G01 machining instruction is started (step sd02). Whether a short-circuit between a wire electrode and a workpiece has occurred or not is determined (step sd03). If a short-circuit has occurred (YES), a program is stopped (step sd04). If a short-circuit has not occurred (NO), movement is made along the G01 machining instruction while electric discharge machining is performed (step sd05). Whether the program has ended or not is determined (step sd06). If the program has not yet ended (NO), the flow returns to step sd02 and the processing is continued (execution of the next machining instruction block is started). If the program has ended (YES), the processing is ended.
In a wire electric discharge machine, when movement is made from the outside of a workpiece to a machining area by the rapid traverse positioning instruction (G00) based on a programmed machining path, the size of the workpiece may be larger than the size assumed when the machining path program is created, and a wire electrode may already cut into the workpiece in an area positioned by rapid traverse.
If the wire electrode cuts into the workpiece in a short-circuit state and the machining instruction (G01) for the next block is issued, electric discharging is not started because of the short-circuit, and the machining instruction (G01) of path information required for tracing back the path is not provided (the positioning instruction (G00) cannot be used to calculate midway movement in the path). Therefore, a short-circuit error occurs and the wire electric discharge machine is brought to a stop state. Particularly, as in polycrystalline diamond (PCD) tool machining, when a PCD tip is brazed to a tool body, a workpiece is often fixed in a position deviated from an assumed workpiece position due to a positioning error of brazing, and the above problem is likely to occur.
When an allowance for the size of a workpiece is made (that is, the size of the workpiece is assumed to be larger to a certain degree), and a machining program is created so that machining is performed significantly before an end face of the workpiece by a machining instruction (such as G01), the above problem does not occur. However, in PCD tool machining, to prevent deterioration of conductivity of a PCD material, it is desirable that a time period in which voltage that is not related to machining is applied is as short as possible. In addition, to reduce operation time, there may be a case in which a wire electrode should be moved to an end face of the workpiece as close as possible by the positioning instruction with a fast movement speed (G00), rather than by the machining instruction with a slow movement speed (G01).
Though a machining is instructed according to the machining instruction (G01) from a state in which a wire electrode is cut into a workpiece and a short-circuit state occurs by a rapid rapid traverse positioning instruction (G00), Such a start of electric discharging in a short-circuit state will causes DC electricity to inevitably flow at once between the wire electrode and the workpiece without resistance of machining fluid in a machining gap between the wire electrode and the workpiece interposed, thereby causing disconnection of the wire electrode. For this reason, to deal with such problem, a wire electric discharge machine is basically designed not to start in a short-circuit state.
To restart machining from this state, a wire electrode position must be temporarily moved to a program start position in the outside of the workpiece by axis movement, and a machining program must be edited, in which the program must be reedited so that the rapid traverse positioning instruction (G00) is used to specify movement to a position in which cutting into the workpiece does not occur, and the machining instruction (G01) is used to specify subsequent movement. Then, the program must be executed again from the beginning.
Each time such a state occurs, the machining program must be edited on a machine or must be reedited on a program device. However, some operators of the machine can perform an operation for starting the machine but may often lack an ability of program editing and have a difficulty in modifying the program. Particularly, if an operator with an ability of program editing is absent during work in the nighttime, the machining work may be started after the operator comes to the office the next day and edits the program, causing a problem of delayed delivery of a product. Therefore, a function is required by which the machining can be restarted from a short-circuit state only by a simple operation, without modifying the machining program.
In the technique disclosed in Japanese Patent Application Laid-Open No. 56-27741 above, a return is made from a machining start point to a halt point along a machining path, but a path movement instruction is only a machining instruction (G01, G02, or G03) and this function is provided for movement in this section by dry-run. Therefore, the function is not a function to be provided in the present invention in which a wire electrode returns while performing electric discharge machining from an arbitrary area other than a machining start point or a midway path to a halt point at which the wire electrode has cut into a workpiece, in a section between a machining start point and a non-machining point of the rapid traverse instruction (G00).
In the technique disclosed in Japanese Patent Application Laid-Open No. 6-277949 above, as in the technique disclosed in Japanese Patent Application Laid-Open No. 56-27741 above, tracing back a path already machined by the machining instruction (G01) is performed. However, a return from a point of the rapid traverse positioning instruction (G00) or an arbitrary area other than the path is not described.
In the technique disclosed in Japanese Patent Application Laid-Open No. 1-210221 above, before the start by program operation, an amount of manual movement from a machining start point to a position in which machining can be performed is stored, and only a return is made backward by the stored movement amount in automatic mode. In this technique, it is not possible to avoid the above problem, that is, a short-circuit state that may occur in a stage in which the machining instruction (G01) is issued after the rapid traverse instruction (G00) following the start of the program.